PROJECT SUMMARY ? PROJECT 2 Small Cell Lung Carcinoma (SCLC) is an aggressive neuroendocrine subtype of lung cancer. SCLC patients have a very low 5-year survival, in large part because SCLC tumors can become rapidly resistant to chemotherapy and radiation therapy and because of a lack of targeted therapies. Emerging evidence supports the idea that, while SCLC tumors seem homogeneous when examined under a microscope, these tumors contain a significant level of intra-tumoral heterogeneity. Indeed, recent observations by our group and others have identified distinct cellular phenotypes in SCLC, including in primary human tumors, in cell lines derived from human tumors, and in tumors from genetically-engineered mouse models. Importantly, data from our group as well as from Project 1 investigators indicate that these cellular phenotypes contribute to SCLC development and potentially response to therapy. The specific goal of this proposal is to elucidate how different cellular subpopulations within SCLC tumors drive SCLC dynamics, growth, survival, and aggressiveness as an ecosystem. To accomplish this goal, we will focus on better understanding the nature of SCLC phenotypic subtypes and how these populations functionally interact with each other and with noncancerous cells in the tumor microenvironment. Specifically, we have previously identified stem-like tumor-propagating cells (TPCs) in SCLC tumors and found that these cells are neuroendocrine and strongly tumorigenic. We have also characterized cell populations derived from these TPCs with distinct phenotypes, including non-neuroendocrine subpopulations that can promote the growth and the spread of the neuroendocrine TPCs. Leveraging these findings as well as our unique genetic mouse models that allow dissection of SCLC phenotype evolution, we will use a combination of experimental and mathematical approaches to investigate how these different SCLC cell types contribute to tumor growth, in relationship with the tumor microenvironment. We will build and use mathematical modeling to predict key interactions between SCLC subpopulations with distinct phenotypes and to uncover fragility/intervention points that could be used for treatment. Modeling will also be a key factor driving experimental design. As part of this design, we will determine how cell-cell interactions in SCLC tumors affect the division and survival rates of the different subpopulations; we will also determine phenotype transition rates between different subpopulations to capture SCLC dynamics and plasticity. Finally, we will elucidate the role of secretory factors released by these SCLC subpopulations in driving survival, growth, phenotype composition, and metastasis of SCLC tumors. These experiments will elucidate basic mechanisms of SCLC development and progression and may ultimately lead to novel therapeutic approaches by identifying key interactions of SCLC subpopulations.